Transmit diversity seeks to vary the transmission path for different aspects of a radio signal. The diversity may be created by sending two signals at different times, on different frequencies, or from different locations. More complex forms of transmit diversity send variations of a single packet more than once so that a receiver may combine the two signals to reconstruct the original packet. In some transmit diversity systems, the two signals both contain all of the information while in other systems, the two signals each contain a different part of the information. Even if all of the bits are not received from one signal or the other, the original packet might be reconstructed using error correction, depuncturing, maximum likelihood sequence estimation or other techniques.
In Long Term Evolution (LTE), transmit diversity for some signals is provided by dividing a packet into parts. The parts are each sent in a different time slot and on a different subcarrier. In addition, different antennas may be used for the two slots. This provides time and frequency diversity and options for spatial diversity.
LTE uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete Fourier transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. FIG. 1 is a grid diagram of the LTE downlink physical resource (3GPP TS 36.211, Third Generation Partnership Project Technical Specification No. 36.211).
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. FIG. 2 is a diagram of the LTE time domain structure where time moves from left to right across the frame.
The resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Downlink (DL) transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information to the remote terminals to indicate the resource blocks assigned to transmit data to each terminal, in the current downlink subframe. The remote terminals are typically referred to in LTE as user equipment or UE. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as control is illustrated in FIG. 3. FIG. 3 is a grid diagram of a DL subframe showing control in the first blocks followed by data traffic, with reference symbols dispersed through the grid.
LTE uses hybrid-ARQ (HARQ), where ARQ refers to Automatic Repeat Request or Automatic Repeat Query, where, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (acknowledgement, ACK) or not (negative acknowledgment, NACK). In case of an unsuccessful decoding attempt, the base station may retransmit the erroneous data.
Uplink control signaling from the terminal to the base station in LTE consists of hybrid-ARQ acknowledgements for received downlink data; terminal reports related to the downlink channel conditions, used as assistance for the downlink scheduling; and scheduling requests, indicating that a mobile terminal needs uplink resources for uplink data transmissions.
If the mobile terminal has not been assigned an uplink resource for data transmission, the L1/L2 (Layer 1/Layer 2) control information (channel-status reports, hybrid-ARQ acknowledgments, and scheduling requests) is transmitted in uplink resources (resource blocks) specifically assigned for uplink L1/L2 control on Release 8 of the Physical Uplink Control Channel (Rel-8 PUCCH).
FIG. 4 is a grid diagram of a PUCCH showing resources assigned for a signal uplink control message. As illustrated in FIG. 4, these resources are located at the edges of the total available cell bandwidth. Each such resource consists of 12 “subcarriers” (one resource block) within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources are frequency hopping on the slot boundary, i.e. one “resource” consists of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa. If more resources are needed for the uplink L1/L2 control signaling, e.g. in case of very large overall transmission bandwidth supporting a large number of users, additional resources blocks may be assigned next to the previously assigned resource blocks.
The PUCCH resource blocks are located at the edges of the overall available spectrum to maximize the frequency diversity experienced by the control signaling. In addition, Assigning uplink resources for the PUCCH at other positions within the spectrum, i.e. not at the edges, would fragment the uplink spectrum, making it impossible to assign very wide transmission bandwidths to a single mobile terminal and still retain the single-carrier property of the uplink transmission
When carrier aggregation is used in LTE, one uplink carrier is designed to carry the HARQ-ACK/NACK bits for all DL carrier Physical Downlink Shared Channel (PDSCH) transmissions. To enable the possibility to transmit more than four bits of ACK/NACK, PUCCH format 3 may be used. The basis for Format 3 is DFT-pre-coded OFDM, as diagrammed in FIG. 5 described below.
If the number of ACK/NACK bits is up to 11, then the multiple ACK/NACK bits (which may also include scheduling request (SR) bits) are Reed-Müller (RM) encoded to form 48 coded bits. The coded bits are then scrambled with cell-specific sequences. 24 bits are transmitted within the first slot and the other 24 bits are transmitted within the second slot. The 24 bits per slot are converted into 12 QPSK symbols, spread across five DFTS (DFT Spread)-OFDM symbols using an orthogonal cover code, DFT pre-coded and transmitted within one resource blocks (bandwidth) and five DFTS-OFDM symbols (time). The spreading sequence is specific to each terminal (UE) and enables multiplexing of up to five users within the same resource blocks.
For the reference signals, cyclic shifted constant amplitude zero autocorrelation (CAZAC) sequences are used. This is shown as a processing diagram in FIG. 5 in which input bits are encoded, scrambled and modulated, then applied to weighting multiplexers and DFTs. The DFTs are applied to inverse fast Fourier transform (IFFT) blocks as shown. This is the DFTS-OFDM based PUCCH format 3 for a UE supporting more than 4 HARQ bits in normal contention period (CP) subframes.
The bandwidth of one resource block during one subframe is too large for the control signaling needs of a single terminal. Therefore, to efficiently exploit the resources set aside for control signaling, multiple terminals may share the same resource block. This is done by assigning the different terminals different orthogonal phase rotations of a cell-specific length-12 frequency-domain sequence and/or different orthogonal time-domain covers covering the subframes within a slot or subframe.
If the number of ACK/NACK bits exceeds 11, then the bits are split into two parts and two RM encoders are used, one for each part respectively. This is known as the dual-RM code. Up to 20 ACK/NACK bits (plus one SR bit) may therefore be supported by PUCCH Format 3. Each encoder in the dual-RM code outputs 24 bits which are converted to 6 quaternary phase-shift keying (QPSK) symbols per slot and the two sets of 6 QPSK symbols are interleaved over the subcarriers so that the first encoder maps its 6 symbols onto odd subcarriers and the second encoder onto even subcarriers. These 12 QPSK symbols are then spread across the five DFTS-OFDM symbols using one out of five orthogonal cover codes, as in the single-RM code case. The encoding and multiplexing are diagrammed in FIGS. 6A and 6B.
FIG. 6A shows encoding and multiplexing up to 11 uplink control information (UCI) bits into slot 0 and slot 1. FIG. 6B shows segmenting 12-21 UCI bits into Segment 1 and Segment 2. These are encoded and mapped to 12 QPSK symbols and then to slot 0 and slot 1.
In LTE Release 10, space orthogonal transmit diversity is used for PUCCH Format 3 to achieve transmit diversity. Transmission with two antenna ports is supported and the Format 3 encoding and mapping shown in FIG. 5 is repeated for each of the two antennas apart from the cyclic shift of the reference signals and the orthogonal cover codes on the data, which are different to provide orthogonality between the antenna ports.